1. Field of the Invention
The present invention relates to a diffusive optical element, an illumination optical system, and an image-shooting apparatus such as a film-reading apparatus.
2. Description of the Prior Art
A film-reading apparatus (film scanner) for reading images recorded on photographic film is provided with an illumination optical system for illuminating the film, a taking lens for imaging the illumination light transmitted through the film, and an image-sensing device for shooting images by receiving the light exiting from the taking lens. The film is disposed on the illumination target plane of the illumination optical system, and the image-sensing device is disposed on the imaging plane of the taking lens that lies conjugate with the illumination target plane.
In a high-performance film-reading apparatus, uniform illumination is essential, and therefore, to make the uneven intensity distribution of a light beam emitted from a light source uniform on the illumination target plane, a Kxc3x6hler illumination optical system provided with an integrator is used. As the integrator, a lens array or rod is used, either of which functions basically in the same manner. Specifically, the light beam from the light source is split into a plurality of light beams, which are then imaged as secondary light-source images in the vicinity of an aperture stop of the taking lens so that the thus split light beams are superimposed on one another on the illumination target plane. This permits every portion on the film to receive all of the split light beams, which makes it possible to realize highly uniform illumination.
From the perspective of efficient use of light, it is preferable to illuminate the film with illumination light that has made into a reasonably parallel beam. However, in a film-reading apparatus, if the film is illuminated with a parallel beam, a scratch or foreign particle, if any, present on the film appears distinctly in the image shot, i.e. read, by the image-sensing device. This can be avoided by disposing a diffusive optical element in the vicinity of the film so that the film is illuminated with illumination light that has a uniform intensity distribution but is nevertheless diffusive.
The more the illumination light is diffused, the more effectively it is possible to reduce the effect on the image shot of a scratch or the like present on the film. For this reason, the diffusive optical element is generally designed to diffuse light in such a way that part of the diffused light reaches outside the area determined by the entrance-side numerical aperture of the taking lens.
A film-reading apparatus employs a light source that emits white light. In general, the intensity of the light emitted from a light source varies greatly from one wavelength range to another. For example, with a halogen lamp, the ratio of the intensity of light it emits in red, green, and blue wavelength ranges is about 10:4:1. Therefore, if the light from such a light source is used intact as illumination light, it is impossible to reproduce correctly the colors of images recorded on film.
This can be overcome by disposing, somewhere in the optical path from the light source to the image-sensing device, a filter that transmits only part of light in a wavelength range in which light intensity is comparatively high so that, in each wavelength range, an amount of light equal to that in the wavelength range in which light intensity is lowest reaches the image-sensing device. For example, with light having an intensity ratio as mentioned above, it is possible to make the amounts of light in individual wavelength ranges uniform by discarding 90% of red light and 75% of green light.
While diffusing illumination light wide is effective in reducing the effect of a scratch or the like present on the film, it simultaneously reduces light use efficiency. In particular when a common diffusion plate formed of frosted glass or of microparticles is used as the diffusive optical element, since such a diffusion plate diffuses light nearly on a perfect-diffusion basis, i.e. in such a way that the intensity of the diffused light is proportional to the cosine of the angle thereof relative to the direction normal to the diffusion plate, while the effect of a scratch or the like present on the film is reduced further, light use efficiency lowers further.
Moreover, a conventional diffusive optical element does not exhibit wavelength dependence, and thus diffuses light of different wavelengths to approximately identical degrees. Therefore, the proportion of light lost through diffusion is constant in all wavelength ranges irrespective of whether light intensity is high or low therein.
FIG. 9 shows the diffusion characteristic of a perfect-diffusion surface as plotted in a polar coordinate system, and FIG. 10 shows the efficiency with which the light from a light source is used in a conventional film-reading apparatus employing a perfect-diffusion surface. As shown in FIG. 10, within a range in which the numerical aperture NA of the taking lens has practical values of 0.2 or less, light use efficiency is about 2% or less.
Low light use efficiency imposes severe restrictions on enhancement of the image-reading performance of a film-reading apparatus. In particular in a scanning-type apparatus that is provided with a line sensor as an image-sensing device and that reads an image line by line, low light use efficiency makes high-speed scanning impossible, and thus imposes a rigid limit on image-reading speed. It is of course possible to enhance image-reading performance to a certain degree by increasing the output of the light source, but doing so leads to higher electric power consumption, and in addition shortens the operating life of the light source.
An object of the present invention is to provide an image-shooting apparatus that excels in light use efficiency, and to provide a diffusive optical element and an illumination optical system suitable for use in such an optical apparatus.
To achieve the above object, according to one aspect of the present invention, a diffusive optical element is designed to diffuse light in such a way that light incident thereon from a predetermined direction is diffused so as to exhibit the maximum intensity in an identical direction irrespective of wavelength and that, the lower the intensity of incident light in a wavelength range, the higher the rate at which the intensity of diffused light in that wavelength range is reduced with increasing angle relative to the maximum intensity direction.
This diffusive optical element diffuses light in such a way that light incident thereon from a predetermined direction is diffused so as to exhibit, after diffusion, the maximum intensity in an identical direction irrespective of wavelength. In addition, light of different wavelength ranges is diffused in such a way that, the lower the intensity of incident light in a wavelength range, the higher the rate at which the intensity of diffused light in that wavelength is reduced with increasing angle relative to the maximum intensity direction. That is, the lower the intensity of incident light in a wavelength range, the narrower the range of angles at which light is diffused in that wavelength range. Accordingly, by using diffused light of different wavelength ranges within an identical range of angles relative to the maximum intensity direction, it is possible to use light of a wavelength range in which light intensity is low (hereinafter called a low-intensity wavelength range) with high light use efficiency and use light of a wavelength range in which light intensity is high (hereinafter called a high-intensity wavelength range) with low light use efficiency.
For example, in cases where, as in a film-reading apparatus, part of light of a high-intensity wavelength range needs to be discarded to make the amount thereof equal to the amount of light of a low-intensity wavelength range, enhancing light use efficiency in a low-intensity wavelength range by the use of the diffusive optical element results in enhancing light use efficiency in all wavelength ranges. On the other hand, reducing light use efficiency in a high-intensity wavelength range by the use of the diffusive optical element has no effect on light use efficiency in all wavelength ranges.
The diffusive optical element may have the shape of a flat plate. In that case, setting the maximum intensity direction to be normal to the diffusive optical element makes it easy to place the element in a desired arrangement relative to another optical element that receives the diffused light. The diffusive optical element can easily be produced as a hologram element.
The rate at which the intensity of the diffused light is reduced with increasing angle relative to the maximum intensity direction may be set separately for light of three wavelength ranges corresponding to red, green, and blue regions of a light spectrum. This makes the element suitable to diffuse white light but nevertheless, by limiting the number of divided wavelength ranges to a small number, it is possible to keep it easy to produce.
According to another aspect of the present invention, an illumination optical system is provided with: a light source that radiates light of wavelengths corresponding to red to blue regions of a light spectrum; an integrator that directs the light radiated from the light source toward a predetermined illumination target plane as a light beam having a uniform intensity distribution; and a diffusive optical element as described above. Here, the diffusive optical element has the rate at which the intensity of the diffused light is reduced with increasing angle relative to the maximum intensity direction set separately for light of a plurality of different wavelength ranges, and the diffusive optical element is disposed in the optical path from the integrator to the illumination target plane in such a way that the light beam exiting from the integrator is incident on the diffusive optical element from the predetermined direction and that the maximum intensity direction is perpendicular to the illumination target plane.
This illumination optical system illuminates the illumination target plane uniformly with diffused white light. The diffusive optical element used here diffuses light, as described above, in such a way that, the lower the intensity of incident light in a wavelength range, the higher the rate at which the intensity of diffused light in that wavelength range is reduced with increasing angle relative to the maximum intensity direction. Thus, the illumination optical system offers high light use efficiency in a low-intensity wavelength range. The diffusive optical element is arranged in such a way that the maximum intensity direction is perpendicular to the illumination target plane, and therefore every point on the illumination target plane receives light with an intensity distribution pattern symmetric with respect to the normal to the illumination target plane through that point.
According to still another aspect of the present invention, an image-shooting apparatus is provided with: an illumination optical system as described above; a taking lens that has an optical axis arranged perpendicularly to the illumination target plane and that images the light beam exiting from the diffusive optical element and transmitted through a subject placed on the illumination target plane onto a predetermined imaging plane; an image-sensing device that is disposed on the imaging plane and that shoots the subject by receiving the light beam exiting from the taking lens; and a light adjuster that adjusts brightness of light differently in different wavelength ranges based on the rates at which the intensity of the diffused light is reduced with increasing angle relative to the maximum brightness direction in the individual wavelength ranges and based on the entrance-side numerical aperture of the taking lens in such a way that the image-sensing device receives light having substantially identical power in the plurality of wavelength ranges.
This image-shooting apparatus shoots as a subject a planar object that transmits light, such as photographic film. The subject is placed on the illumination target plane, and every portion thereof is illuminated uniformly with diffused white light. Of the light emitted from the light source, light in a low intensity wavelength range is incident on the subject at incident angles within a comparatively narrow range, and light in a high-intensity wavelength range is incident thereon at incident angles within a comparatively wide range. Thus, more light enters the taking lens in a low-intensity wavelength range than in a high-intensity wavelength range, which enhances light use efficiency. A scratch or foreign particle present on the subject deflects the path of the illumination light and thereby reduces the amount of light that enters the taking lens from that spot. However, since light in a high-intensity wavelength range is incident on the subject at incident angles within a wide range, even if its path is deflected by a scratch or foreign particle, the greater part thereof enters the taking lens. This helps prevent a scratch or the like present on the subject from appearing distinctly in the image shot by the image-sensing device.
The light adjuster adjusts the brightness of light differently in different wavelength ranges in such a way that the image-sensing device receives light having equal power in a plurality of wavelength ranges. This adjustment is performed based on the rates at which the intensity of the diffused light is reduced with increasing angle relative to the maximum brightness direction in the individual wavelength ranges and based on the entrance-side numerical aperture of the taking lens. As a result of this brightness adjustment, the unnecessary portion of light of a high-intensity wavelength range is discarded, which makes insignificant the fact that the diffusive optical element exhibits lower light use efficiency in a high-intensity wavelength range than in a low-intensity wavelength range. The light adjuster may be disposed anywhere in the optical path from the light source within the illumination optical system to the image-sensing device.
The image-shooting apparatus described above may be so configured as to have a flat-plate-shaped diffusive optical element arranged parallel to the illumination target plane and in addition fulfill expression (1) below.
1/[2 tan(xcex8NA)]xe2x89xa6Lxe2x89xa6DF/[2 tan(2 xcex8NA)]xe2x80x83xe2x80x83(1) 
where L represents the distance from the diffusive optical element to the illumination target plane, DF represents the effective diameter of the subject, and xcex8NA represents the arcsine of the entrance-side numerical aperture of the taking lens.
Expression (1) defines the permissible range of the distance from the diffusive optical element to the illumination target plane in terms of the effective diameter of the subject and the entrance-side numerical aperture of the taking lens. If the distance from the diffusive optical element to the illumination target plane is longer than the upper limit defined by expression (1), light use efficiency tends to be low; by contrast, if the distance is shorter than the lower limit defined by expression (1), a foreign particle, if any, present on the diffusive optical element tends to appear distinctly in the image shot.